RETRACTION
I thought I knew what Zach was saying, but he's since gotten better at saying it. So I'm issuing a retraction.
Zach has not been saying, "big fat bum." He has been saying, "spank that bum."
I discovered this tonight when, as usual, he rolled over on his changing table and stuck his diapered bum up in the air. He said it a little more clearly this time, but I knew I'd finally gotten it right when I did "spank that bum," and was rewarded with squeals of joy.

But he's come up with two more sayings that tests my straight facedness (no, that's not a word) to the max. He gets this very stubborn and irritated look on his face when Jared is bugging him, sets his jaw, and says, "No funny, Jee jee!" usually while taking a swipe at him. He doesn't get bullied around!
And the other one is that he tells Jared off when he's making some awful sound (a favorite past time of his) by saying, "Stop, Jee jee, NOYing!" (annoying).

He's a hoot and a half!